Loans - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) Item	Mar. 31, 2015 USD ($)	Feb. 29, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration of loans in a single industry in excess of ten percent	0.00%
Maximum percentage loan related to single industry	10.00%
Percentage of total energy loans to total loans	14.40%			15.30%
Troubled debt restructurings	$ 61,347	$ 613
Number of loans restructured during the last year in excess of 90 days past due | Item	1
Previously restructured loans past due	$ 17
Minimum number of days to be considered past due	90 days
Texas Leading Index economic condition indicator			120.6	123.0
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	$ 61,095	0
Non-Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	61,300
Additional interest that would have been recognized on non-accrual loans, net of tax	844	$ 397
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,300,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 61,100